Slow wave structure for travelling wave tubes



J. FROOM June 28, 1966 SLOW WAVE STRUCTURE FOR TRAVELLING WAVE TUBES 2 Sheets-Sheet 1 Filed July 9, 1964 F/Gf.

PRIOR ART Inventor JOCELYN FRQOM June 28, 1966 J. FROOM 3,258,722

SLOW WAVE STRUCTURE FOR TRAVELLING WAVE TUBES Filed July 9, 1964 2 Sheets-Sheet 2 2c 50 2 \E i 5 f-2o b-l F/gi WK .4. -4- N: 5 T :1 T

Inventor JOCEL YIV FROOM ttorne y United States Patent 3,258,722 SLKEW WAVE STRUCTURE FOR TRAVELLING WAVE TUBES Jocelyn Froorn, Bishops Stortford, Herttordshirc, England, assignor to International Standard Electric (Zorporation, New York, N.Y., a corporation of Delaware Filed duly 9, N64, Ser. No. 381,499 Claims priority, application Great Britain, Aug. 16, 1963, 32,476/63 2 tClairns. (Cl. 3333ll) The invention relates to travelling wave tubes and is particularly concerned with the construction of slow wave structures for use in such tubes.

According to the present invention there is provided a slow wave structure for a travelling wave tube including a pair or" parallel opposed metal side walls and a slotted metal sheet joined at right angles to each of the side walls so that the side walls extend above and below the sheet, the said sheet having a set of similar transverse slots extending alternately from one or the other side wall beyond the centre of the sheet so as to form a stubsupported meander line, the portions of the sheet beyond the end of each slot and the opposite side wall being left unslotted.

An embodiment of the invention will be described with reference to the accompanying drawings in which:

FIG. 1 is a diagrammatic view in perspective of part of a prior art meander-line slow wave structure;

FIG. 2 is a plan view of a slotted metal sheet of the present invention for use in the structure of FIG. 1; and

FIG. 3 is a transverse cross sectional view of a slow wave structure according to the invention using two of the sheets of FIG. 2.

A very satisfactory slow wave structure for use in travelling Waves of either the forward or backward wave type and known as a stub-supported meander line consists essentially of a pair of metal side walls parallel and opposite one another, between which are stretched a set of similar transverse conducting strands. These strands are joined together in consecutive pairs by short longitudinally extending conducting strands disposed alternately to one side and the other of a median plane parallel to the side walls. A zigzag conducting path is thus formed by the longitudinal and transverse conducting strands from one end to the other of the slow wave structure. A convenient known way of producing a stub-supported meander line is illustrated in FIG. 1, in which a metal sheet 1 is pierced by a number of long and short slots so that the conducting strands referred to above are formed by the metal between the slots. Side walls 2 and 3 are each formed in two portions such as 2a and 2b, and the metal sheet l is sandwiched between them.

A suitable novel form for the metal sheet as used in the present invention is shown in FIG. 2. Unpierced marginal portions 4 are left on either side of the sheet for clamping between the side walls of the slow wave structure. A set of long transverse slots 5, extending alternately from each side, is cut in the metal sheet between the marginal portions 4. The slots 5 extend from one marginal portion beyond the centre line of the sheet with the short slots 6 of FIG. 1 being eliminated so as to leave a wide longitudinal connecting link 7 between each pair of transverse bars 8 which are formed in the sheet by the slotting process. It will be seen that the set of longitudinal connecting links 7 together with the transverse bars 8 are equivalent to the conducting strands of the prior art stub-supported meander line first mentioned, and provide a zigzag conducting path from end to end of the slow wave structure.

In the drawings the slot widths have been considerably exaggerated to show the pattern of the meander line. In a typical metal sheet for use at 6 millimetres wavelength the distance between the marginal portions 4, that is, the distance between the side walls 2 and 3 is 0.140 inch, the bars and slots are each of width 0.0054 inch, the slots 5 of length 0.0955 inch, While the thickness of the metal sheet is 0.0025 inch. The sheet is made of molybdenum and the slots are formed by a photo-etching technique. Alternatively the slots may be produced by a spark-erosion process.

Considerations of current flow along the meander line have led us to the conclusion that very little disturbance to the electric field configuration should result if the slots 6 of FIG. 1 are omitted. This is confirmed by experiment. In embodiments of the present invention therefore the short slots 6 are in fact omitted from the sheet 1 so that the slot attern is given by the full lines in FIG. 2. The simpler resulting slot pattern is more easy to produce by spark-machining and, moreover, has rather greater thermal dissipation than the fully slotted sheet.

FIG. 3 shows diagrammatically one form of use of a slow wave structure according to the invention. The side walls 2 and 3 are each formed in three parts to a, b, c and two identical metal sheets i and la are sandwiched between the parts b and c, and a and b, respectively. The sheets I and 1a are parallel and close together with the slots in each aligned one above the other. In the completed travelling wave tube a ribbon-shaped electron beam 3 is projected between them. The side walls 2 and 3 project above and below the sheets 1 and 1a, respectively, for the electro-magnetic fields of the structure to be negligible at the top and bottom of the side walls.

It is to be understood that the following description of specific examples of this invention is not to be considered as a limitaion on its scope.

What is claimed is:

I. A slow wave structure for a travelling wave tube including a pair of parallel opposed metal side walls and a longitudinal slotted metal sheet joined at right angles to each of the side walls so that the side walls extend above and below the sheet, the said sheet having a set of similar transverse slots extending alternately along the length of the sheet from one side wall and the other side wall beyond the center of the sheet so as to form a stub-supported meander line, the portions of the sheet between the end of each transverse slot and the opposite side wall being left unslotted.

2. The slow wave structure of claim 1 wherein the marginal portions of the sheet are clamped between aligned longitudinally extending solid metal members providing a pair of side walls.

References Cited by the Examiner UNITED STATES PATENTS 3,181,090 4/1927 Ash 3l53.5X

HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner. 

1. A SLOW WAVE STRUCTURE FOR A TRAVELLING WAVE TUBE INCLUDING A PAIR OF PARALLEL OPPOSED METAL SIDE WALLS AND A LONGITUDINAL SLOTTED METAL SHEET JOINED AT RIGHT ANGLES TO EACH OF THE SIDE WALLS SO THAT THE SIDE WALLS EXTEND ABOVE AND BELOW THE SHEET, THE SAID SHEET HAVING A SET OF SIMILAR TRANSVERSE SLOTS EXTENDING ALTERNATELY ALONG THE LENGTH OF THE SHEET FROM ONE SIDE WALL AND THE OTHER SIDE WALL BEYOND THE CENTER OF THE SHEET SO AS TO FORM A STUB-SUPPORTED MEANDER LINE, THE PORTIONS OF THE SHEET BETWEEN THE END OF EACH TRANSVERSE SLOT AND THE OPPOSITE SIDE WALL BEING LEFT UNSLOTTED. 